The present invention relates to an image display apparatus, e.g., a liquid crystal display apparatus, used in a television receiver. It finds particular, in a liquid crystal television receiver that uses the so-called "double line sequential interlace" scanning system, in which the scanning lines on a screen of one field are doubled. This doubles the number of displayed pixels in the vertical direction of the screen are doubled. The apparatus generates a display of (i) television images on an interlace scanning system, (ii) a display of characters and graphics on a non-interlace scanning system, and a display by force of a character and graphic image superimposed on a television image without deteriorating the quality of the picture image. Although the preferred embodiment is explained below in conjunction with a liquid crystal display, by way of example, it is to be appreciated that the invention is also applicable to other video and graphics display devices.
Japanese Laid-open Patent Publication No. 63-26084, "Double Line Sequential Scanning Circuit", discloses a circuit a liquid crystal display panel is provided with liquid crystal display elements or the elements are pixels arranged in a matrix array at the intersecting points of a plurality of horizontally extended scanning electrodes and a plurality of vertically extended signal lines. The display elements are adapted to be driven when both of the electrodes are driven simultaneously. It is thereby made possible to have a television signal (hereinafter sometimes simply referred to as "TV signal") on an interlace scanning system displayed on the panel without deteriorating the vertical resolution of the image.
More specifically, in the aforesaid prior art, the same horizontal scanning signal is stored in two sample-and-hold circuits, for example. The signal electrodes arranged in the vertical direction on the liquid crystal display panel are sequentially driven by the stored signals, while two lines of the liquid crystal display elements on the panel screen are scanned in one horizontal scanning period of the television signal. During this scanning operation, the combination of two lines in the first field of the television signal and the combination of two lines in the second field are shifted with respect to their phases. In this manner, double line sequential scanning is achieved.
FIG. 1a to FIG. 1c are drawings which illustrate scanning in such a double line sequential interlace scanning system.
FIG. 1a is a schematic diagram showing ordinary interlace scanning of a TV signal. If it is assumed that scanning lines 23H, 24H, 25H, . . . written in solid lines are scanning lines in the first field (hereinafter sometimes referred to as "ODD field"), then the broken lines 285H, 286H, 287H, . . . represent the scanning lines in the second field (hereinafter referred to as "EVEN field"). In this case, the scanning lines in the ODD field and the scanning lines in the EVEN field are inserted between each other (i.e., shifted with respect to their phases) so that the resolution in the vertical direction is enhanced.
FIG. 1b is an explanatory diagram of the double line sequential interlace scanning system on a liquid crystal panel screen.
Referring to FIG. 1a, if it is assumed that there is an image expressed by the slant-lined band covering both the scanning line numbers 23H and 286H, the image, when displayed on the liquid crystal panel screen on the double line sequential interlace scanning system, becomes as shown in FIG. 1b.
More specifically, in the ODD field in FIG. 1b, the horizontal scanning line 23H, which should originally be that for one horizontal line, is used two times for scanning the first line (L1) and second line (L2). By so doing, the number of scanning lines in the ODD field is doubled, i.e. becomes equal to the sum total of scanning lines in one frame.
The same is true for the EVEN field. In the EVEN field, the second line and third line, shifted downward by one line from those in the ODD field on account of the interlace scanning system, are scanned two times by the 286th horizontal scanning line. However, similarly to the above, the number of the scanning lines in one field is doubled.
In the liquid crystal panel screen using the double line sequential interlace scanning system, problem arise when still pictures, i.e. character and graphic data are displayed rather than television pictures with brisk movements.
As an example of such character and graphic data, consider the one image line shown by the slant-lined band shown in FIG. 1a, for example, Referring now to FIG. 1b, since the one line is displayed according to the double line sequential interlace scanning system in this case, the one line is displayed as two lines along the first and second lines in the ODD field. The same one line is also displayed as two lines along the second and third lines in the EVEN field as described above.
As a result, the line is widened or stretched vertically. When the image is viewed during one frame period, the one line is displayed along three lines on the screen, namely, the first line on the screen (during the ODD field period), the second line on the screen (during the ODD field and EVEN field periods), and the third line on the screen (during the EVEN field period). Then, the resultant image originally of one sharp line becomes a thick three line image with blurred edged. Therefore, a problem arises in that the resolution of the image is deteriorated.
Further, if the three lines are examined from the point of view of the driven liquid crystal display elements on the screen, then, as shown in FIG. 1b, the first line is driven in the first field and not driven in the second field. The second line is driven in both the fields. The second fields, and the third line is not driven in the first field and is driven in the second field. That is, symmetrical driving is performed along the second line during the period of the first and second fields but asymmetrical driving is performed along the first line and the third line.
During asymmetrical driving, a D.C. component remains applied to the liquid crystal display elements. This causes flickering on the screen (as described in the reference, Papers to be Read on the 14th Forum on Liquid Crystal, 2B109 (1988)). Also, there was a problem of sticking or ghosting, i.e., a fixed pattern remaining observable on the screen due to existence of internal electric field even after the applied D.C. component has disappeared.
One prior art technique for preventing the trouble was to adopt a non-interlace scanning method. More specifically, a signal on the non-interlace system was input as a character and graphic signal. The same first and second lines were driven to be displayed as two lines in both the ODD field and in the EVEN field as shown in FIG. 1c.
In the above described prior art, no consideration was made of a superimposed display of a character and graphic signal (a signal on a non-interlace scanning system) on a TV signal for moving pictures (a signal on an interlace scanning system).